chore(writer): enable native writer by default (#15278)

## Description
Enables the native writer by default. The native writer can still be
disabled through _DD_TRACE_WRITER_NATIVE.

## Testing
All tests are run with the native writer. The native writer have also
been enabled on some services internally for dogfooding purposes.

## Risks
This changes the logic behind trace export and client-side stats
computation (for customers who enabled it). We expect no change in
behavior for trace exporting. Client-side stats behavior may show some
differences as the current python implementation is not fully compliant
with the client-side stats spec.

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Co-authored-by: Emmett Butler <[email protected]>
(cherry picked from commit c2e0ab9)

Author: 2 people